The present invention relates to a process and apparatus for drying water-containing materials. More particularly, the present invention relates to a process and apparatus for drying materials with internally generated superheated steam in a drying zone while preventing ambient air from entering such drying zone. In one aspect, the present invention relates to a method and apparatus for drying thin sheet materials such as wood veneer. In another aspect the invention relates to a process for drying particulate materials.
Wood veneer is normally dried by passing large volumes of hot air over the veneer sheets. The water content of the hot air is usually about 5 percent, but may be as high as 30 percent or more in unusual cases. When overly hot air contacts the veneer, the surface of the veneer begins drying immediately and very rapidly dries out completely, becoming overly hot, while the interior of the veneer is still relatively cool and moist. This temperature gradient, and the resulting moisture gradient in the veneer cause hardening, cracking and general degradation of the veneer. For this reason, the wood veneer drying art has resorted to relatively low drying air temperatures and long drying times.
It is desirable to have a uniform moisture content in the veneer after drying is completed. In conventional veneer drying the outer surface of the veneer is overly dry after drying is completed, while the interior of the veneer remains overly moist. Moreover, sheets of veneer dried at different times in the same dryer often have different moisture contents, making it difficult to glue the veneer uniformly to produce plywood.
A major problem in the veneer drying art has been disposal of large amounts of hydrocarbonaceous vapors which are evolved from the veneer during drying. The hydrocarbonaceous vapors are removed from the veneer by hot air in the dryer and carried out of the dryer up a smoke stack along with the air. In the past, after the hot air has been used in drying, it has simply been discarded up the stack and released into the atmosphere. In addition much of hydrocarbon vapor leaks out of door seals and cracks in the veneer dryer as "fugitive emissions" because of the positive pressure created within the dryer. This has caused severe air pollution problems. Under present environmental restrictions, it has become necessary to curb release into the atmosphere of such hydrocarbonaceous vapors.
Drying of particulate materials such as seeds, alfalfa, manure, etc., has also been accomplished by contacting the materials with large volumes of hot air. Typically, air has been taken from the atmosphere and heated, contacted with the material to be dried, and then simply released back into the atmosphere. Heat energy imparted to the air before it is used in drying is thus lost when the air is released. Air drying of particulates, like air drying of veneer, has created air pollution problems as a result of production of various volatile materials stripped from the drying particulate material by the hot drying air, which have simply been released into the atmosphere along with used air.
Conventional air drying of particulates is relatively uneconomical in that the heat energy imparted to air used in drying is wasted. Further, it has been found difficult and expensive to remove volatile pollutants from the heated air after use in drying, before the air is returned to the atmosphere.
These problems of pollution control and energy waste are overcome by the steam drying method and apparatus of the present invention. While superheated steam has been used previously in batch drying systems for conditioning and drying heavy lumber and has been suggested for increased humidity to prevent fires in the operation of veneer dryers along with reduced air in flow for heat conservation, as discussed by S. E. Corder in Forest Products Journal, October 1963, pages 449 to 453, it has not been employed along with means for producing a negative internal pressure within the drying chamber in continous production drying systems such as those used for drying wood veneer or particulates to prevent the fugitive emission of pollutants from such chamber into the plant containing such dryer.